Belt pressing of placing a continuous long pressed object on a press belt and pressing the pressed object between a first pressing member located inside the periphery of the press belt and a second pressing member located outside the periphery of the press belt is generally performed in various types of industries. The term “pressing member” denotes a press roll, a pressure shoe or the like. A shoe press employed as a dehydrating press in the paper industry can be listed as an exemplary belt press.
Simply stated with reference to the paper industry, the shoe press is employed for a method of performing pressing (dehydration) by applying an area pressure to a pressed object (wet web) placed on the outer peripheral surface of a press belt through the press belt between a press roll employed as external pressing means located outside the periphery of the press belt and a pressure shoe employed as internal pressing means located inside the periphery of the press belt. While a roll press performing pressing with two rolls applies a linear pressure to the pressed object, the shoe press can apply an area pressure to the pressed object with the pressure shoe having a prescribed width in the traveling direction. When dehydration pressing is performed with the shoe press, therefore, a nip width can be advantageously increased for improving dehydration efficiency.
A shoe press roll prepared by covering a pressure shoe serving as internal pressing means with a flexible cylindrical press belt (press jacket) and assembling the same into a roll in order to compactify a shoe press has come into wide use, as typically disclosed in Patent Document 1, for example.
In addition to the aforementioned dehydration step, shoe pressing may be performed in place of roll pressing or along with roll pressing in a calendering step carried out for smoothing the surface of a pressed object and putting a gloss thereon in order to improve the quality of the pressed object in the paper industry, the magnetic recording medium manufacturing industry, the textile industry etc., for example. Strength, wear resistance, flexibility and nonpermeability with respect to water, oil, gas etc. can be listed as general characteristics required to a press belt. Polyurethane obtained by reacting a urethane prepolymer and a hardener with each other is generally used for the press belt as a material having these characteristics. However, severe bending or pressing is repeated on the press belt, particularly the shoe press belt, and hence the outer periphery of the press belt is easily cracked to result in a serious problem in durability.
As a method of solving the aforementioned problem, Patent Document 2 discloses a shoe press belt improved in wear resistance and cracking resistance by varying the hardness of resin constituting the belt to be high on a cross-directional central area and low on both edge areas including portions corresponding to shoe edges. In this case, the belt conceivably has an effect of maintaining wear resistance and pressure deflection resistance on the central area while rendering the both edge areas hardly crackable.
Cracking is easily concentrically caused on terminal corresponding areas corresponding to both cross-directional ends of pressing means such as a press roll or a pressure shoe. It is conceivable that not very strict cracking resistance is required but wear resistance and pressure deflection resistance must rather be emphasized on a central area located between the terminal corresponding areas for serving as a pressing surface for a pressed object.
While Patent Document 2 has been proposed on the basis of this idea, hardness must be varied to some extent on the central area and the both edge portions in order to compatibly attain wear resistance and cracking resistance by varying the hardness. Contracting force in molding varies with the hardness of polyurethane. When the hardness is remarkably varied on the central area and the both edge portions of the belt, therefore, cylindricity may be deteriorated to hinder the traveling property.
On the other hand, Patent Document 3 discloses a method of preventing cracking by preparing the composition of polyurethane constituting the outer peripheral surface of a press belt so that the equivalent ratio (H/NCO) between an active hydrogen group (H) of a hardener and an isocyanate group (NCO) of a urethane prepolymer is 1<N/NCO<1.15.
According to this method, the overall press belt can be inhibited from cracking. When the equivalent ratio is set to 1<N/NCO<1.15, however, wear resistance of the overall press belt is disadvantageously reduced. Particularly in a shoe press belt employed for a dehydrating press of a paper machine, cavities such as grooves or blind holes for draining are formed on the outer peripheral surface of the belt in a sheet width range allowing passage of a wet web, and the cavities are reduced in capacity to reduce drainability when the press belt is worn or pressure-deflected.
Patent Document 1: Japanese Patent Laying-Open No. 61-179359
Patent Document 2: Japanese Patent Laying-Open No. 10-298893
Patent Document 3: Japanese Patent Laying-Open No. 2002-146694